Method of and device for equalizing solidification process of molten metal produced, in particular, during strip casting

ABSTRACT

A method of equalizing a solidification process of molten metal produced, in particular, during strip casting, includes subjecting the molten metal to an electromagnetic stirring process, with a magnetic field is applied to the molten metal located upstream of the area of the electromagnetic stirring process. The electromagnetic stirring includes applying, during the solidification process, at least one electromagnetic field to an already solidified, external region ( 11 ) of the molten metal ( 10 ).

RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No. 12/734,786 filed Jun. 17, 2010 which is a national phase application of PCT application PCT/EP2008/09938 filed Nov. 24, 2008 which claims priority of German application DE 10 2007 059 919 filed Nov. 26, 2007, all three applications are incorporated herein by reference thereto.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method of and a device for equalizing solidification of molten metal produced, in particular, during strip casting and wherein the molten metal is subjected, in particular, to an electromagnetic stirring process, wherein a magnetic field is applied to the molten metal, upstream of the area of, in particular, electromagnetic stirring. The invention also relates, in addition, to a device for carrying out the method.

2. Description of the Prior Art

Basically, during a solidification process of a cast strip, e.g., on a cooled conveyor belt that transports the cast strip, large heat removal takes place at the bottom of the cast strip, with additional heat removal at the upper surface and the narrow sides, mostly by radiation. As a result, a non-uniform vertical temperature profile is formed over the strip cross-section and which creates, during a subsequent cooling process, stresses in the strip which can lead to warping of the strip. Because the contact of the cast strip and the conveyor belt, in particular, in width direction, is not constant and the removal of heat from the cast strip over its width is not uniform, it again leads to a non-uniform solidification structure.

Further, the non-uniform distribution of the temperature in a plane perpendicular to the casting direction results in the cast strip being displaced on the conveyor belt in the casting direction in form of U-shaped groove, with bulging edges lifted off the conveyor belt.

With the middle part of the cast strip lying on the cooled conveyor belt, the strip is cooled under shrinkage. The lifted side portions of the strip are cooled only by heat radiation. The different type of cooling of the middle and side portions of the strip leads to a more slow shrinkage of the edge portions of the strip in comparison with the middle portion of the strip. The tensions induced by different shrinkage of portions of the strip lead to formation of fissure in the edge portions which, as a result, assume a somewhat waved profile.

In this connection, the prior art discloses different methods and devices with electromagnetic stirring in the region of liquid steel melt. By way of example, reference is made to the following publications:

U.S. Pat. No. 4,933,005 discloses an induction stirring method according to which, molten metal is electromagnetically stirred usually with intensity that produces turbulence in the molten metal to impart fluidity to melt-solid suspensions, and upstream of the area of the electromagnetic stirring, a static magnetic field is applied to the molten metal for reducing turbulence in order to minimize the meniscus distortions.

The described method is directed to improving induction stirring applications where, among others, a free surface exists during stirring in molds and during electromagnetic stirring in ladles or other containers, and surface disturbances and distortions in meniscus should be reduced to a minimum.

Japanese Publication JP 06182502 A relates to a single metal strip-type caster in which it is suggested to provide an electromagnetic brake above a region of a molten metal and specifically at the drawing side of the metal belt rather than at a point of pouring molten metal on the belt, in order to prevent waving of the molten metal region and to obtain a metal strip with a flat surface without roughness. At the time of pouring of the molten metal from a tundish on a metal belt, waving is generated on the surface of the molten metal region by the pouring flow of the molten metal. The electromagnetic brake is provided above the molten metal regions and, in particular, at the drawing side of the metal belt rather than at the point of pouring molten metal. This arrangement eliminates waving in the direction of the metal belt as seen from the electromagnetic brake, and so a flat molten metal is formed. Therefore, due to the formation of a solidified shell without waving in the molten metal region, a flat surface shape of the surface of the solidified shell, without roughness, is achieved.

With regard to these two publications, it can be said that the known methods and devices cannot prevent the above-described problems of warping and fissures formation.

The object of the invention is to improve and further develop the known methods and devices, while retaining the existing advantages, that the above-mentioned drawbacks are eliminated, wherein, in particular, optimization of a precise shape of a strand, a better control of a metallurgical length, and a better adaptation of the speed are achieved.

SUMMARY OF THE INVENTION

According to the inventive method, the object of the invention is achieved by applying, during the solidification process, at least one electromagnetic field to an already solidified, external region of the molten metal.

Thereby, in a simple way, the naturally developed temperature profile in a still molten core is homogenized by action of an electromagnetic field on the already solidified external region of the melt. The uniform distribution of energy in the molten core provides always the highest possible temperature at the inner side of the strip shell. As a result, the thickness growth of the shell is delayed, whereby the heat removal is increased. Due to a higher heat removal, the cast strip solidifies more rapidly. Overall, by producing a uniform temperature field over the cross-section of the molten core, the shell is heated somewhat again at the start of the electromagnetic stirring, so that its thickness growth is delayed, and the shell, which remains warm longer and becomes thinner, only later acquires its mechanical characteristics. Thereby, it lies flatly on the cooled conveyor belt for a longer time which leads to reduction of inner stresses and of a possible high-arching of the edges.

The electromagnetic stirring of already solidified external region of the molten metal, thus, permits to prevent wobbling during transportation of the cast strip on a horizontal conveyor belt without using an upper conveyor belt, by providing a uniform temperature profile in a plane extending perpendicular to the casting direction. The stirrer primarily generates a traveling electromagnetic field.

In the absence of the upper conveyor belt, the electromagnetic stirring is applied to an upper or top surface of the cast strip.

According to an embodiment of the inventive method, the electromagnetic field can be applied to an already solidified, external region of the melt essentially at a bottom of the molten metal. The electromagnetic stirring can be applied to both the upper and bottom surfaces of the strip.

According to a further feature of the inventive method, a position of the electromagnetic stirring process is adapted in the casting direction. The stirrer is displaceable in the casting direction in accordance with the position of the “kissing point” which depends on a casting speed. At a lower speed, the “kissing point” is located further upstream on the conveyor belt than it is located at a greater speed. Accordingly, the stirrer, which is always located at a certain distance upstream of the “kissing point,” would also be located further upstream with regard to the conveyor belt, than at a greater speed.

According to the invention, the inventive device includes means for applying, during the solidification process, at least one magnetic field to an already solidified, external region of the molten metal. With respect to the advantages achieved thereby, in order to prevent repetition, reference is made to the described advantages of the method.

According to embodiments of the inventive device, the device is arranged so that at least one electromagnetic field is applied to solidified, external region of the molten metal either at a top or bottom of the molten metal, or both.

Further advantages and particularities of the invention follow from the dependent claims and the following description in which an embodiment of the invention, which is shown in the drawings, is explained in detail. In addition to the above-described combination of features, separate features or in other combinations form an essential part of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings show:

FIG. 1 a schematic view of a temperature profile of the molten metal and of the strip shell with the use of the inventive method in comparison with the state of the art;

FIG. 2 an enlarged view of the left portion in FIG. 1;

FIG. 3 a schematic view of solidified lengths of the molten metal in the casting direction; and

FIG. 4 a schematic view illustrating the position of an electromagnetic stirrer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The inventive method serves for equalizing the solidification process of molten metal which is designated with 10 in FIG. 1 and which is produced, in particular, during casting of a strip. The molten metal 10 is subjected to an electromagnetic stirring process, with a braking static magnetic field being applied to the metal upstream of the area of the electromagnetic stirring. According to the inventive method, during the solidification process, at least one traveling electromagnetic field is applied to an already solidified, external region 11 of the molten metal 10.

The electromagnetic field of the stirrer is so formed that a distribution of melt particles take place in the vertical plane extending perpendicular to the horizontal conveyor belt, in particular, with cooler particles at the bottom of the strip moving upward in somewhat cooler upper regions of the strip and warmer particles moving downward from cooler upper regions of the strip in the warmer bottom regions. This distribution of the particles advantageously leads to a more rapid growth of the shell at the top of the strip, which reduces bulging of the cast strip displaceable on the conveyor belt.

The electromagnetic stirring is applied at a location of the belt where a complete shell has already been formed on the strip upper, bottom and side surfaces of the strip, but with a liquid core remaining, i.e., before the “kissing point.”

This inventive feature provides the previously described advantages. A further advantage consists in that by producing a uniform temperature field over the cross-section of the molten core, the strand shell 12 is heated somewhat again at the start of stirring so that its thickness growth is delayed, and the shell 12, which remains warm longer and becomes thinner, only later acquires its mechanical characteristics. Thereby, it lies flatly on the cooled conveyor belt for a longer time which leads to reduction of inner stresses and of a possible high-arching of the edges. FIGS. 1 and 2 show a temperature profile of the molten metal 10 and the strip shell 12 with the use of the inventive method in comparison with the state of the art. It can be seen that the solidified region 11 of the metal 10 and, thus, the thickness d₂ of the strip shell 12 is substantially thicker than the solidified region 13 of the metal 10 at a delayed solidification and, thus, the thickness d₁ of the strip shell 12, see FIGS. 1 and 2. It is further shown the temperature profile of the molten metal 10 and the strip shell 12 wherein the temperature with stirrer shows the temperature of the stirring process, and the temperature without stirrer shows the temperature without the stirring process. It can be seen that the temperature without the stirring process raises noticeably faster and, finally, is at a higher level than the temperature with the stirring process.

According to embodiments of the inventive process, the electromagnetic field is applied to the already solidified, external region 11 of the molten metal 110, at top and/or bottom of the molten metal 10. It can further be provided to adapt the position of the electromagnetic stirring process in the casting direction in accordance with the casting speed.

FIG. 3 shows solidification lengths of the molten metal 10 in the casting direction shown with arrow A. The solidification length EN shows a normal solidification length, and the solidification length EV shows a shortened solidification length with stirring.

The present invention also relates to a device, see FIG. 4, for carrying out the method, in particular, the electromagnetic stirring process in the molten metal 10. The device includes an adjustable brake/stirrer unit for applying at least one traveling electromagnetic field during the stirring process to an already solidified, external region 11 of the molten metal 10. The position of the electromagnetic stirring process can be adapted to the casting direction in accordance with the cast speed. The stirrer can be displaced either automatically or manually. 

1. A method of equalizing a solidification process of a cast strip transported on a horizontal conveyor belt, comprising the steps of: pouring a molten metal on a horizontal cooled conveyor belt from one of horizontal tundish and a horizontal cast mold; applying a magnetic field to the molten metal; and applying, downstream of a point of application of the magnetic field, a stirring electromagnetic field to already solidified external region of the molten metal.
 2. A method according to claim 1, wherein the applied magnetic field is a static magnetic field and the applied stirring electromagnetic field is a traveling electromagnetic field.
 3. A method according to claim 1, wherein the step of applying the stirring electromagnetic field includes applying the electromagnetic field at a top surface of the molten metal.
 4. A method according to claim 1, wherein application of the electromagnetic field takes place at start of stirring.
 5. A method according to claim 1, further comprising the step of applying a further stirring electromagnetic field at a bottom surface of the molten metal.
 6. A method according to claim 1, wherein a position of an electromagnetic stirring is adapted, in a casting direction, in accordance with a cast speed.
 7. A device for equalizing a solidification process of a cast strip transported on a horizontal conveyor belt, comprising: means for subjecting the molten metal to an electromagnetic stirring process, means for applying a magnetic field to the molten metal; and stirring means located downstream of the magnetic means, for applying, during the solidification process, at least one electromagnetic field to an already solidified, external region of the molten metal.
 8. A device according to claim 7 wherein stirring means is arranged at a top of the molten metal.
 9. A device according to claim 7, wherein a position of the stirring means is adapted, in a casting direction, according to the cast speed of the molten metal. 